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SQL MERGE — One Statement for INSERT + UPDATE + DELETE
`MERGE` (standard SQL, PostgreSQL 15+, SQL Server, Oracle) is a single statement that combines INSERT/UPDATE/DELETE driven by joining a source against a target. Heavy artillery for reconciliation jobs.
Java CountDownLatch — Wait for N Events
Block one or more threads until a count of events occurs. Set the count up front; each event calls `countDown()`; waiters call `await()`. Classic "wait until all workers are ready" pattern.